Demagnetizing Factor Corrections for Densely Packed Single-Domain Magnetic Nanoparticles

Single-domain magnetic nanoparticles are routinely characterized with magnetometry. Often, test samples are prepared as densely packed powders. To calculate the true or intrinsic properties of the particles from measured data, the sample geometry needs to be correctly accounted for. However, these corrections are often ignored. Moreover, the proper demagnetizing correction factors for packed assemblies of single-domain magnetic particles are not well agreed upon. For non-particulate, solid ferromagnetic materials, the intrinsic properties of the measured material can be calculated using suitable demagnetizing field correction factors (samples are usually approximated as solid ellipsoids). For particulate single-domain nanomagnets, however, the appropriate choice for demagnetizing correction factors is less obvious: the literature has suggestions based on effective medium theories and the physics of single-domain nanomagnets. We performed magnetometry on disk-shaped samples containing densely packed 5 nm, single-domain, superparamagnetic iron oxide nanoparticles and calculated the intrinsic properties of the particles using different correction factors. We show that proper selection of demagnetizing factor is critical for calculating several properties of the particles, including the Curie-Weiss interaction temperature and the remanent magnetization at low temperature.